The present invention relates to polypropylene multi-layer sealant films for retort packaging, and more particularly, it relates to polypropylene multi-layer sealant films which are used by lamination with heat-resisting films for retort packaging involving heat sterilization.
Conventional retort-packed products, particularly retort-packed food, usually contain liquid food, such as cooked curry, causing no problem that the inner layers of a packaging material in the retort-packed products may adhere to each other (often referred to as the xe2x80x9cblockingxe2x80x9d) by accident. For this reason, in the laminated films consisting of heat-resisting film layers and sealant layers for retort packaging, the sealant layers may usually be formed from polypropylene films, particularly those of propylene random copolymers or propylene block copolymers. Also known are sealant films of the above polymers containing ethylene-propylene rubber or ethylene-butene rubber mixed therein. However, with the diversification of processed food, retort-packed food has further include various kinds of food without excess water content, such as hamburger and sausage.
Many kinds of meat-processed food, such as hamburger, sausage, and ham, contain no excess liquid flowing around the food. When the former polypropylene films are used for sealant layers, there arises a problem that the sealant layers may cause a phenomenon (often referred to as the xe2x80x9cretort blockingxe2x80x9d) of their adhering (or fusing) to each other at their directly facing portions without these kinds of food intervening therebetween. The retort blocking can be solved by addition of anti-blocking agents to the sealant layers. In the step of lamination with heat-resisting films, however, when the laminated films are rolled up, there arises blocking of the rolled films (often referred to as the xe2x80x9claminate blockingxe2x80x9d), even if anti-blocking agents are used as described above, causing a problem, such as variation of tension in unrolling, in the pouch-making process using the rolled films. Further, the retort blocking may be prevented, possibly by the increased roughness on the surface of sealant layers in the laminated films for retort packaging. In this case, however, the other surface of these sealant layers also has increased roughness, and therefore, in the step of lamination with heat-resisting films, air bubbles remain left at the interface between the sealant layers and the heat-resisting films, resulting in poor appearance of the laminated films as packaging materials and thereby making the laminated films unsuitable for practical use.
In the method of spraying silicone-coated starch powder to prevent the laminate blocking of laminated films with heat-resisting films, many problems arise because silicone-coated starch powder is very fine, including deterioration of working conditions in the step of lamination, poor appearance of retort pouches, and floating of powder in the step of food filling.
Under these circumstances, the present inventors have extensively studied sealant films for retort packaging. As a result, they have found that the control of surface roughness on each surface of a sealant film by taking a multi-layer structure in the sealant film makes it possible to solve the above problems in the prior art, thereby completing the present invention.
Thus, the present invention provides polypropylene multi-layer sealant films for retort packaging, each comprising a multi-layer film composed mainly of at least one polypropylene polymer and to be laminated on one surface of a heat-resisting film, the multi-layer film having a first surface facing the heat-resisting film when laminated therewith and further having a second surface on the other side, the first surface having a surface roughness as defined below in (1), and the second surface having a surface roughness as defined below in (2):
(1)
(i) center-line mean roughness (Ra): 0.020 to 0.250 xcexcm;
(ii) maximum height (Rmax): 0.3 to 2.9 xcexcm; and
(iii) ten-point mean roughness (Rz): 0.2 to 2.4 xcexcm;
(2)
(i) center-line mean roughness (Ra): 0.050 to 0.300 xcexcm;
(ii) maximum height (Rmax): 3.0 to 13.0 xcexcm; and
(iii) ten-point mean roughness (Rz): 2.5 to 11.5 xcexcm.
The polypropylene multi-layer sealant films of the present invention can cause no formation of air bubbles at the interface when bonded with heat-resisting films, thereby giving excellent laminate appearance, and can avoid blocking of the sealant films facing each other to cause their fusion after rolling up or after retort packaging.
In a preferred embodiment, the above polypropylene polymer may be a propylene random copolymer or a propylene block copolymer.
In another preferred embodiment, the above multi-layer film may have at least one layer containing at least one thermoplastic elastomer.